1. Field of the Invention
The present invention relates to an image-forming apparatus for forming images using toner with substantially the same hue and different densities, and its control method.
2. Description of the Related Art
An image-forming apparatus for forming images by an electrophotographic system includes a charging unit for uniformly charging a photosensitive surface of a photosensitive drum; a latent-image forming unit for forming latent images on the charged photosensitive surface corresponding to image information; a developing unit for developing the latent images with developer; a transfer unit for transferring the developed latent images on a recording material; and a fixing unit for fixing the transferred images on the recording material.
Developer (toner) used generally includes a single kind with a predetermined density for colors such as cyan, magenta, yellow, and black. However, when such a single-kind of density toner is used, in a highlight portion (low-density portion), a toner amount is reduced, so that there has been a difficulty with reproducibility in gradation (gray scale) of image data. Recently, users' demand level has been increased so that image-forming apparatuses using developer with the increased number of colors in comparison with a conventional image forming apparatus using four-color toner have been proposed. That is, an electrophotographic image-forming apparatus using toner with substantially the same hue and different densities is proposed in Japanese Patent Laid-Open No. 2001-290319 and No. 2004-145137.
Such image-forming apparatuses frequently use six-color developer for light cyan and light magenta as well as cyan, magenta, yellow, and black. The developer for light cyan and light magenta contains less pigment than that for normal cyan and normal magenta although its spectral characteristics are the same. The developer for normal cyan and normal magenta is referred to as dark toner and the developer for light cyan and light magenta is referred to as light toner herein. An image signal using dark toner during developing is referred to an image signal for dark toner while an image signal using light toner is referred to as an image signal for light toner.
FIG. 19 is a graph showing the relationship between image densities of image signals for dark toner and light toner and a toner coating weight/an output density. Solid line T1 and dotted chain line T2 in the graph show the relationship between the image densities of the image signals and the light toner coating weight/dark toner coating weight on recording sheets, respectively. Straight line m shows characteristics between the image density of the input image signal and the ideal output density, and the light toner coating weight/dark toner coating weight versus the image densities of the input image signals are determined so that the output density of the images formed with the light toner and the dark toner becomes ideal linear. If the maximum value of the image density of the input image signal is 1.8, over a region from a highlight portion (low density portion) being below 0.9 to an intermediate density portion, images are formed with only the light toner for reducing the image granulated effect. Over a region from the intermediate density portion being above an image density of 0.9 toward the high density, images are formed with both the dark toner and the light toner by suppressing the amount of the light toner.
However, in an image-forming apparatus for forming images using the dark toner and the light toner, the output characteristics of the dark toner and the light toner are changed, problems arise as follows.
For example, if the resistance of the surface layer of the photosensitive drum or the charged amount (tribo) of developer is reduced, the contrast potential (Vcont) is decreased, resulting in reduction of the output density due to changes in the toner coating weight.
This point will be described more in detail. Curved line I shows that the dark toner coating weight is reduced, and characteristics of the output density at this time are shown in curved line n. As is apparent from curved line n, in the intermediate density portion (about 0.9 image density) where images are started forming with the dark toner, the output density is rapidly changed. Accordingly, in the images using the intermediate density, the gradation may become artificial or a false outline may be generated.
In an image-forming apparatus using six-color developer (six-color toner), it is desirable to switch the image-forming mode between the high image-quality mode forming images with six colors and the normal mode using only four fundamental colors giving priority to reduction in toner consumption and improvement in the image-forming speed, in accordance with user's demands. Hence, in order to switch the operating mode between four-color output and six-color output in one apparatus, it is necessary to adjust the gradation and density so as to have optimum printed images in the respective four-color output and the six-color output.